Visual processing is mediated by precise neural connections organized in a hybrid parallel and heirarchical architecture. Parallel magno (M)- and parvocellular (P) streams are segregated in early visual pathways, and provide input to separate populations of neurons in primary visual cortex (V1). Within V1, however, information from these two pathways is integrated to give rise to output neurons influenced by both. The proposed work is aimed at obtaining a detailed understanding of the circuits mediating these interactions. What are the relationships between the M and P pathways and the neurons that project from V1 to higher , extrastriate visual cortical areas? The flow of information will be studied within V1, from layers 4Calpha and 4Cbeta (which receive M and P input respectively) to 3 major populations of extrastriate projection neurons in layer 4B, layer 2/3 "blobs , layer 2/3 interblobs . These studies will also assess the extent of interactions between neurons in blobs and the specificity of these horizontal" connections for ocular dominance columns. All of the studies will utilize intracellular labeling of individual neurons with biocytin during whole- cell recording in living slices of V1. Subsequent anatomical reconstructions will identify the locations of their synapses. In particular, the positions of synaptic boutons from individual layer 4B will be determined. Similar measures will reveal the specificity of horizontal connections from layer 2/3 neurons. Understanding the mechanisms mediating normal visual perception is crucial to identifying processing defects in human patients suffering from strabismus and amblyiopia. These studies may also shed light on causes of dyslexia, which is linked to deficits related to the M pathway.